This invention relates to a process for the surface treatment of non-consumable electrodes, current collectors and other metallic components of electrochemical cells to improve the corrosion resistance of such components and to increase their surface area and modify the surface texture, thereby enhancing the adherence of coatings applied to the surface, including pastes and active materials.
Electrochemical cells utilizing numerous electrodes and current collectors are used in a variety of commercial applications. Non-consumable electrodes, current collectors or other metallic components find use in the electrowinning of metals, in other electrochemical processing applications, gas diffusion electrodes used in metal-air batteries, fuel cells and gas sensors. They include the use of non-consumable electrodes such as DSA (dimensionally stable anodes) used in a number of electrochemical synthesis applications e.g. electrowinning of metals practiced on a large scale in the production of electrorefined zinc and copper. Cobalt, nickel, chromium, manganese, cadmium, gallium, thallium, indium, silver and gold have been reported to also be produced using electrolytic processes.
Electrolysis is also used to manufacture sodium, potassium or ammonium salts of several peracids including persulfates, perchlorates, periodates and perborates by anodic oxidation of sulfate, chlorate, iodate and borax respectively, and electrolysis is the only commercial process used for several of these anions. The electrochemical formation of all these peroxyanions requires high positive electrode potentials in typically acidic electrolytes. Anodes typically comprise platinum or lead-dioxide coatings on a base metal collector. Electrochemical oxidation is also used to produce permanganates at temperatures of about 60xc2x0 C. in caustic electrolytes, using nickel or monel anodes.
Other electrochemical devices include storage batteries employing current collectors to retain the active mass and provide the electric conductivity required to charge/discharge the plates.
This invention describes a novel surface treatment of non-consumable electrodes, current collectors or other metallic components for use in electrochemical cells to enhance corrosion resistance and adhesion. The treatment consists of peening the surface of the article, or a precursor of the article; for instance peening a metal or metal alloy strip prior to punching or expanding it into a battery grid. To enhance the corrosion resistance further the peening can be followed by a heat treatment. Additional improvements to the corrosion resistance can be achieved if the peening/annealing treatments are repeated. The peening also modifies the surface texture, enlarging the surface area and enhancing adhesion of active materials or coatings.
To enhance the longevity of non-consumable electrodes, current collectors and other metallic components for use in electrochemical cells a variety of metal, metal alloys and composites have been developed. In many applications the environment to which the metallic articles are exposed is highly corrosive. Research has been carried out to find ways of enhancing the stability, e.g. by reducing the corrosion-induced weight loss and growth. These can become a problem, particularly when the component is exposed to oxidizing potentials.
The prior art describes the use of coatings on current collectors and non-consumable electrodes to enhance stability and longevity. In the case of storage battery electrodes, the active material or paste needs to adhere to the collector to maintain good electrical contact throughout the service life. To enhance adhesion of coatings and pastes means of modifying the surface of current collectors and non-consumable electrodes have been proposed.
Mao in U.S. Pat. No. 3,929,513 (1975) describes a corrosion resistant lead-alloy product for use in a lead-acid battery having a thin surface layer created by heat-treatment and following quenching of an article manufactured using pressure casting.
Prengaman in U.S. Pat. No. 3,953,244 (1976) describes stable wrought lead-calcium-tin alloys which are prepared by casting, cold working the casting, preferably using rolling to one quarter of the original thickness, within two to three days after casting and heating aged work pieces sufficiently to dissolve the precipitated calcium phases.
Johnson in U.S. Pat. No. 4,483,785 (1979) described an improved current collector and/or container for use in high temperature battery applications comprising a non-corrosive, conductive ceramic member and a conductive metal cladding attached to a substantial portion of the surface.
Kim in U.S. Pat. No. 4,978,601 (1990) discloses a method of laser treating lead-containing battery grids that relies on virtually instantaneous melting and solidification of the surface, thereby yielding an improved fine-grained microstructure improving mechanical and corrosion resistant properties.
Fiorino in U.S. Pat. No. 5,521,029 (1996) describes a current collector for a lead-acid battery, wherein the substrate is the current collector coated with conductive titanium suboxides to enhance the corrosion resistance.
Muller in U.S. Pat. No. 5,593,798 (1997) describes a method of producing corrosion resistant electrodes and other surfaces in corrosive batteries using ion-implantation.
Yu-Lin in U.S. Pat. No. 5,858,575 (1999) describes a method for extending the high temperature cycle-life of a lead-acid battery positive electrode current collector by immersing the lead-alloy mesh in a lead-antimony or lead-silver molten metal bath to apply a coating.
Palumbo et al. in International Publication No. WO 99/07911 describe a method for processing a lead-based alloy electrowinning electrode material to improve its properties by a repetitive sequence of cold deformation and recrystallization heat treatment steps, within specified limits of deformation, temperature and annealing times, which has the effect of significantly increasing the frequency of xe2x80x9cspecialxe2x80x9d grain boundaries in the microstructure of the electrode material.
The present invention affords a novel surface treatment for non-consumable electrodes, current collectors and like components of electrochemical cells, which comprises peening the surface of the component, or a precursor of the electrochemical cell component, as by peening a metal or metal alloy strip prior to punching or expanding it into a battery grid.
Peening consists of directing a stream of shot at high velocity on the surface of the metal component under controlled conditions. As such, it is a special method of cold working, which induces compressive stresses confined to the surface layer of a metallic article.
Peening has been used for the purpose of relieving tensile stresses that contribute to stress-corrosion cracking in metallic articles. Yamada in U.S. Pat. No. 5,816,088 describes a surface treatment method for a steel workpiece using high speed shot peening. In U.S. Pat. No. 5,932,120 (Mannava) there is described a laser shock peening apparatus which employs a low energy laser.
We have discovered that the application of peening to a component of an electrochemical cell, such as a non-consumable electrode, or to an immediate precursor of such a component, not only enhances corrosion resistance in the chemical environment encountered by such components, but also improves the surface adhesion to the component of active materials or coatings used in the electrochemical cell. By following the peening operation with an appropriate heat treatment, the corrosion resistance of the cell component may be enhanced still further.
It is an object of the present invention to provide electrochemical cell components, in particular non-consumable electrodes and current collectors, which exhibit a high degree of chemical stability and good adhesion properties.
It is a further object of the present invention to provide a process for treatment of a finished electrochemical cell component which improves its operating properties without substantial dimensional deformation of the component.
It is a still further object of the present invention to modify the surface texture of an electrochemical cell component so as to increase the surface area and improve the adhesion thereto of coatings and pastes.
With a view to achieving these objects there is provided a method for treatment of the surface of a metallic component or precursor thereof used in an electrochemical cell, comprising shot, laseror hammer peening said component. Optionally, the peening step may advantageously be followed by annealing the component at a temperature below its melting point.